There certainly are quite a few differences in aesthetic objectives with respect to modern dance and classical ballet. Aesthetics can be understood as anything that is "thought of as beautiful," while beautiful can be understood as anything "moving or stirring to the senses" ("A simple Discussion of the Aesthetics of Ballet and Modern Dance"). Hence, while there are differences in aesthetic objectives, the central aesthetic objective for both modern dance and ballet would remain the same--to move or stir the senses.
Modern dance has certainly progressed since the movement first began in the late 19th and early 20th centuries. At first, modern dance was choreographed to include movements that seemed "natural" or "organic," and the dancers had "bare legs" and "bare feet" and merely wanted to express themselves through movement ("A Simple Discussion"). However, today, modern dance has progressed so much that it becomes difficult to precisely define exactly what aesthetic objectives modern dance has. In addition, variations in modern dance is a main point of the objective of modern dance, leading to a variety of aesthetics. However, one aesthetic goal we can state is that a dancer's body must look "interesting and expressive" as the dancer completes his/her motions ("A Simple Discussion"). The dancer's body must also look healthy overall. Regardless, the emergence of community dance within modern dance has also introduced a wide variety of body types within dance. Community dance has been designed to reflect all of what we would see in a community--everything from thin and athletic to obese and even elderly.
In contrast, ballet has developed a very specific aesthetic with respect to what a dancer's body should look like. Choreographer George Blanchine of the de Basil and Blum's Ballet Russe de Monte Carlo introduced in 1932 ballet's current body-type trend when he hired three very young dancers of the ages 12, 13, and 14. Since then, it has become understood that a woman's body type for classical ballet should be "extreme thinness, long limbs, short torso, short of stature, high arches of the feet, and flexibility" ("A Simple Discussion"). Hence, while the body type for modern dance can be extremely varied as a part of the aesthetic object, for ballet, the aesthetic body type is very, very specific. What's more, while a ballerina must be extremely strong, her body type must also look very frail.
The aesthetics of ballet and jazz dance are also very antithetical. Ballet likes to focus on linear movements through the "erect spine and lifted center," while jazz is performed with bended knees and a dropped center to "get into the movement" (Yeuell, "Jazz Dance"). The aesthetic objective of jazz is generally to look more "released and grounded" (Yeuell). In contrast, for ballet, the shapes are drawn through taught muscles, strong lines, and flexibility.
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